The present invention relates to a fuel assembly used for a core of a D-lattice type boiling water reactor (hereinafter, referred to as a xe2x80x9cBWRxe2x80x9d), a reactor core using the fuel assembly, and a fuel spacer and a channel box used for the fuel assembly.
A gap structure between fuel assemblies, called a D-lattice structure, has been used for a core of a BWR. In the D-lattice core, a gap between the adjacent two of the fuel assemblies on the side where a control rod for power adjustment is inserted (on the control rod side) is wider than a gap between the adjacent two of the fuel assemblies on the side where the control rod is not inserted (on the anti-control rod side).
For a BWR, water in a core acts as a coolant and a neutron moderator. In general, the moderating action for neutrons becomes large in a region in which water entirely, continuously exists, and accordingly, water between fuel assemblies plays a large role for moderating neutrons.
One important factor associated with a reactor core is a linear heat generation rate. The maximum linear heat generation rate, which is the maximum value among the linear heat generation rates in a reactor, becomes important for design of the reactor. If the maximum linear heat generation rate becomes excessively large, the center temperature of the corresponding fuel rod becomes excessively high. In this case, there is a possibility that the thermal integrity of fuel pellets and a cladding tube constituting the fuel rod cannot be ensured. From the viewpoint of the safety of a reactor, a specific limited value of the maximum linear heat generation rate is determined. To keep a thermal margin state of a reactor, the maximum linear heat generation rate may be desirable to be made as small as possible.
One important factor associated with a fuel assembly is a fuel assembly critical power. In a core of a BWR, water flows in the lower portion of a fuel assembly, being boiled while flowing in the vicinity of fuel rods, and flows out of the upper portion of the fuel assembly. It may be considered that, at the upper portion of the fuel assembly, the rate of steam becomes large and the surfaces of the fuel rods are covered with liquid films. As the power of the fuel assembly is increased, the liquid film on the surface of any one of the fuel rods is initially lost by evaporation. The power at the time when the liquid film is initially lost is called the critical power. The critical power varies depending on the flow rate of coolant flowing through the fuel assembly. A reactor is operated while it is usually checked that the power of each of the fuel assemblies is less than the critical power.
Taking into account the above-described circumstances, a fuel assembly has been designed such that a suitable fuel enrichment distribution is set by preparing a plurality of kinds of fuel pellets or a suitable concentration distribution of burnable poison added to fuel rods is set, to make small a xe2x80x9crelative axial peaking factor of a fuel assemblyxe2x80x9d or a xe2x80x9clocal peaking factorxe2x80x9d which is the relative power peaking of each fuel rod in a cross section of the fuel assembly, thereby improving the critical power characteristic, and enhancing the safety margin and fuel economy of the reactor.
In a D-lattice core, as described above, there is a difference between a gap between the adjacent two of the fuel assemblies on the control rod side and a gap between the adjacent two of the fuel assemblies on the anti-control rod side. During usual operation, since most of the control rods are pulled out, the effect of moderating neutrons on the side where a gap between the adjacent two of the fuel assemblies is wide (on the control rod side) is larger than that on the side where a gap between the adjacent two of the fuel assemblies is narrow (on the anti-control rod side). When fuel assemblies are loaded in the D-lattice core, the power obtained from a fuel rod near a wide gap between the fuel assemblies (on the control rode side) is different from that obtained from a fuel rod near a narrow gap between the fuel assemblies (on the anti-control rode side). Accordingly, the value of the local peaking factor becomes relatively larger and thereby the maximum linear heat generation rate tends to be made larger. As a result, the above-described fuel enrichment distribution or the concentration distribution of burnable poison must be finely set, so that the degree of freedom in design of fuel assemblies is reduced.
To solve the above problem, the structure called a C-lattice has been proposed. In the C-lattice core, since the gap between the fuel assemblies on the control rod side is equal to that on the anti-control rod side, the degree of freedom in design of the C-lattice core becomes larger than that of the D-lattice core. To be more specific, in the C-lattice core, it is possible to relatively easily obtain the optimum structure in terms of energy efficiency. For example, the discharge exposure of fuel (energy obtainable from fuel in a unit weight) in the C-lattice core can be larger several percentage than that in the D-lattice core. In this way, the C-lattice core is superior to the D-lattice core in terms of fuel economy.
However, since there have been a number of functioning D-lattice cores, attempts have been made to improve these D-lattice cores for enhancing the fuel economies thereof. One of such prior arts has been disclosed in Japanese Patent No. 2791132. The prior art provides a fuel assembly for a D-lattice core including fuel rods placed in a square lattice array of 9xc3x979 (9-rows/9-columns), in which the fuel rod pitch is reduced, and a distance between the outermost fuel rod and a channel box on the control rode side is made smaller than a distance between the outermost fuel rod and a channel box on the anti-control rode side, whereby a difference in gap between fuel assemblies on the control rod side and the anti-control rod side is made small. With this configuration, it is possible to make the core characteristic of such a D-lattice core close to that of a C-lattice core while adopting the same fuel rods and control rod drive mechanism as those having been used in the conventional D-lattice core.
The above-described prior art D-lattice core, however, has the following problem. Since the fuel rod pitch becomes small, cooling water less flows between the fuel rods, to reduce the heat removal performance by cooling water. As a result, it is difficult to ensure the thermal margin at the same linear heat generation rate. To be more specific, the thermal margin of the fuel assemblies in the D-lattice core according to the above-described prior art is made smaller than that of the fuel assemblies in the conventional D-lattice core.
Another problem of the above-described prior art D-lattice core is as follows: namely, in general, fuel rods and water rods placed in a square lattice array are bundled with fuel spacers at a plurality of axial positions and the fuel spacers each have holding members (for example, cylindrical members) for holding the fuel rods and water rods such that they are spaced from each other at specific gaps, and therefore, if the fuel rod pitch is changed, the pitch of the holding members must be correspondingly changed. In other words, according to the prior art D-lattice core, the existing fuel spacers cannot be used and new fuel spacers must be used.
An object of the present invention is to provide a fuel assembly for a D-lattice core, which is capable of achieving the fuel economy comparable to that of a C-lattice core without reducing the thermal margin, and of using the existing fuel spacers.
(1) The present invention provides a fuel assembly including a plurality of fuel rods placed in a square lattice array of 9-rows/9-columns and at least one water rod, wherein the fuel rod pitch is in a range of 14.15 mm to 14.65 mm; and means for offsetting and holding a fuel bundle composed of the fuel rods and the water rod is provided in such a manner that the center in a cross section (cross section center) of the fuel bundle is offset from the center in a cross section of a lower tie plate toward the channel fastener side.
In this document, xe2x80x9cfuel bundlexe2x80x9d is not the same as xe2x80x9cfuel assemblyxe2x80x9d. xe2x80x9cFuel bundlexe2x80x9d includes the fuel rods and the water rod(s), but does not include the channel box, and so on.
In a D-lattice core, upon arrangement of fuel assemblies, a gap between the adjacent two of the fuel assemblies on the control rod side (channel fastener side) is wider than a gap between the adjacent two of the fuel assemblies on the anti-control rod side (anti-channel fastener side). Accordingly, a continuous water region on the channel fastener side on which the gap between the fuel assemblies is wide is larger than that on the anti-channel fastener side on which the gap between the fuel assemblies is narrow, so that the effect of moderating neutrons on the channel fastener side is larger than that on the anti-channel fastener side. As a result, on the channel fastener side, the power obtained from a fuel rod becomes relatively large and thereby the local peaking factor tends to become relatively large.
According to the present invention, the center in a cross section of the fuel bundle is offset toward the channel fastener side by the offsetting/holding means. To be more specific, it is possible to get almost the same effect as follows, the continuous water region on the anti-channel fastener side is made large and simultaneously the continuous water region on the channel fastener side is made small. With this configuration, it is possible to reduce the difference between the continuous water regions on both the channel fastener side and the anti-channel fastener side. This makes it possible to lower the difference in power of fuel rods between the channel fastener side and the anti-channel fastener side, and hence to reduce the local peaking factor. In this way, according to the D-lattice core using the fuel assembly of the present invention, it is possible to obtain a core characteristic comparable to that of a C-lattice core.
(2) The present invention also provides a fuel assembly including a plurality of fuel rods placed in a square lattice array of 9-rows/9-columns and at least one water rod, wherein the fuel rod pitch is in a range of 14.15 mm to 14.65 mm; and means for offsetting and holding a fuel bundle composed of the fuel rods and the water rod is provided in such a manner that the center in a cross section of the fuel bundle is offset a value Yxe2x89xa72xe2x88x923/2 mm from the center in a cross section of a lower tie plate toward the channel fastener side.
When the center in a cross section of the fuel bundle is offset a value Yxe2x89xa72xe2x88x923/2 mm toward the channel fastener side, the fuel bundle is moved by 0.25 mm or more in the row or column direction of the square lattice array. With this configuration, the local peaking factor can be reduced by at least 1% or more as compared with the prior art fuel assembly in which the fuel bundle is not offset, and accordingly, it is possible to certainly obtain a core characteristic comparable to that of a C-lattice core, and hence to improve the fuel economy.
(3) In the configuration described in the item (2), the value Y may be preferably in a range of 7xc3x972xe2x88x921/2 mmxe2x89xa7Yxe2x89xa72xe2x88x923/2 mm.
In the case of the fuel assembly including fuel rods placed in a square lattice array of 9-rows/9-columns, as described in xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d, vol. 43, No. 530 (September, 1988; Wilmington Business Publication) p12-31, the diameter of a fuel rod is generally about 11.0 mm. To ensure the thermal margin, it is required to set a gap between the adjacent fuel rods at about 3 mm. In this case, a distance between both ends of the nine pieces of the fuel rods becomes 123 mm (11.0xc3x979+3xc3x97(9xe2x88x921)=123), and the inner width W of a channel box which surrounds the fuel bundle is about 134 mm.
Accordingly, the remaining distance between the fuel rods positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box on both sides is 11 mm at maximum (134xe2x88x92123=11). Meanwhile, a gap of 2 mm or more is generally required between the fuel rod positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box because a band of each fuel spacer must be inserted in the gap. It is generally required to give the same gap from the viewpoint of the thermal margin.
Accordingly, the actually usable remaining distance between the fuel rods positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box for offsetting the fuel bundle becomes 7 mm (11xe2x88x922xc3x972=7) To be more specific, when the fuel bundle is offset on the channel fastener side, the actually movable maximum distance in the row or column direction of the square lattice array becomes 7 mm. According to the present invention, on the basis of the above knowledge, the center in a cross section of the fuel bundle is offset a value Yxe2x89xa67xc3x972xe2x88x921/2 mm on the channel fastener side. At this time, the movement distance of the fuel bundle in the row or column direction of the square lattice array becomes a value of 7 mm or less.
(4) The present invention provides a fuel assembly including a plurality of fuel rods placed in a square lattice array of 9-rows/9-columns and at least one water rod, wherein the fuel rod pitch is in a range of 14.15 mm to 14.65 mm; each of fuel spacers includes a plurality of tabs for offsetting and holding a fuel bundle composed of the fuel rods and the water rod in such a manner that the center in a cross section of the fuel bundle is offset from the center in a cross section of a lower tie plate toward the channel fastener side, the tabs being provided in such a manner as to project outwardly from the outer periphery of said fuel spacer; and a distance L1 between the leading end of one of said plurality of tabs positioned on the channel fastener side and one of said fuel rods positioned at the outermost periphery of said square lattice array, and a distance L2 between the leading end of one of said plurality of tabs positioned on the anti-channel fastener side and one of said fuel rods positioned at the outermost periphery of said square lattice array are determined in such a manner as to satisfy a relationship of L2xe2x88x92L1xe2x89xa70.5 mm.
(5) In the configuration described in the item (4), said distances L1 and L2 may be preferably determined in such a manner as to satisfy a relationship of 7.0 mmxe2x89xa7L2xe2x88x92L1xe2x89xa70.5 mm.
(6) The present invention also provides a fuel assembly including a plurality of fuel rods placed in a square lattice array of 9-rows/9-columns, at least one water rod, and a channel box provided in such a manner as to surround a fuel bundle composed of the fuel rods and the water rod, wherein the fuel rod pitch is in a range of 14.15 mm to 14.65 mm; the inner width of said channel box is in a range of 133.5 mm to 134.5 mm; and means for offsetting and holding the fuel bundle is provided in such a manner that the center in a cross section of the fuel bundle is offset from the center in a cross section of the channel box toward the channel fastener side.
(7) The present invention also provides a fuel assembly including a plurality of fuel rods placed in a square lattice array of 10-rows/10-columns and at least one water rod, wherein the fuel rod pitch of is in a range of 12.65 mm to 13.15 mm; and means for offsetting and holding a fuel bundle composed of the fuel rods and the water rod is provided in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of a lower tie plate toward said channel fastener side.
(8) The present invention also provides a fuel assembly including a plurality of fuel rods placed in a square lattice array of 10-rows/10-columns and at least one water rod, wherein the fuel rod pitch is in a range of 12.65 mm to 13.15 mm; and means for offsetting and holding a fuel bundle composed of the fuel rods and the water rod is provided in such a manner that the center in a cross section of said fuel bundle is offset a value Yxe2x89xa72xe2x88x923/2 mm from the center in a cross section of said lower tie plate toward said channel fastener side.
(9) In the configuration described in the item (8), said value Y may be preferably in a range of 7xc3x972xe2x88x921/2 mmxe2x89xa7Yxe2x89xa72xe2x88x923/2 mm.
In the case of the fuel assembly including fuel rods placed in a square lattice array of 10-rows/10-columns, as described in xe2x80x9cnuclear engineering INTERNATIONALxe2x80x9d, vol. 43, No. 530 (September, 1988; Wilmington Business Publication) p12-31, the diameter of a fuel rod is generally about 10.05 mm. To ensure the thermal margin, it is required to set a gap between the adjacent fuel rods at about 2.5 mm. In this case, a distance between both ends of the ten pieces of the fuel rods becomes 123.0 mm (10.05xc3x9710+2.5xc3x97(10xe2x88x921)=123.0), and the inner width W of a channel box which surrounds the fuel bundle is about 134 mm.
Accordingly, the remaining distance between the fuel rods positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box on both sides is 11 mm at maximum (134xe2x88x92123=11). Meanwhile, a gap of 2 mm or more is generally required between the fuel rod positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box. Accordingly, the actually usable remaining distance between the fuel rods positioned at the outermost periphery of the square lattice array and the inner peripheral surface of the channel box for offsetting the fuel bundle becomes 7 mm (11xe2x88x922xc3x972=7). According to the present invention, on the basis of the above knowledge, the center in a cross section of the fuel bundle is offset a value Yxe2x89xa67xc3x972xe2x88x921/2 mm on the channel fastener side. At this time, the movement distance of the fuel bundle in the row or column direction of the square lattice array becomes a value of 7 mm or less.
(10) The present invention also provides a fuel assembly including a plurality of fuel rods placed in a square lattice array of 10-rows/10-columns, at least one water rod, fuel spacers for holding a plurality of axial positions of a fuel bundle composed of the fuel rods and the water rod, wherein the pitch of said plurality of fuel rods is in a range of 12.65 mm to 13.15 mm; each of said fuel spacers includes a plurality of tabs for offsetting and holding said fuel bundle in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of a lower tie plate toward said channel fastener side, said tabs being provided in such a manner as to project outwardly from the outer periphery of said fuel spacer; and a distance L1 between the leading end of one of said plurality of tabs positioned on the channel fastener side and one of said fuel rods positioned at the outermost periphery of said square lattice array, and a distance L2 between the leading end of one of said plurality of tabs positioned on the anti-channel fastener side and one of said fuel rods positioned at the outermost periphery of said square lattice array are determined in such a manner as to satisfy a relationship of L2xe2x88x92L1xe2x89xa70.5 mm.
(11) In the configuration described in the item (10), said distances L1 and L2 may be preferably determined in such a manner as to satisfy a relationship of 7.0 mmxe2x89xa7L2xe2x88x92L1xe2x89xa70.5 mm.
(12) The present invention also provides a fuel assembly including a plurality of fuel rods placed in a square lattice array of 10-rows/10-columns, at least one water rod, and a channel box provided in such a manner as to surround a fuel bundle composed of the fuel rods and the water rod, wherein the pitch of said plurality of fuel rods is in a range of 12.65 mm to 13.15 mm; the inner width of said channel box is in a range of 133.5 mm to 134.5 mm; and means for offsetting and holding said fuel bundle is provided in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of said channel box toward said channel fastener side.
(13) The present invention also provides a reactor core including a plurality of fuel assemblies each of which includes a plurality of fuel rods placed in a square lattice array of 9-rows/9-columns, and at least one control rod inserted among said fuel assemblies, said plurality of fuel assemblies being configured such that a gap between the adjacent two of said plurality of fuel rods on the control rod side is larger than a gap between the adjacent two of said plurality of fuel rods on the anti-control rod side, wherein at least one of said plurality of fuel assemblies is configured such that the pitch of said plurality of fuel rods is in a range of 14.15 mm to 14.65 mm; the inner width of a channel box is in a range of 133.5 mm to 134.5 mm; and means for offsetting and holding a fuel bundle is provided in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of said channel box toward the channel fastener side.
(14) The present invention also provides a reactor core including a plurality of fuel assemblies each of which includes a plurality of fuel rods placed in a square lattice array of 9-rows/9-columns, and at least one control rod inserted among said fuel assemblies, said plurality of fuel assemblies being configured such that a gap between the adjacent two of said plurality of fuel rods on the control rod side is larger than a gap between the adjacent two of said plurality of fuel rods on the anti-control rod side, wherein at least one of said plurality of fuel assemblies is configured such that the pitch of said plurality of fuel rods is in a range of 14.15 mm to 14.65 mm; the inner width of a channel box is in a range of 133.5 mm to 134.5 mm, and means for offsetting and holding a fuel bundle is provided in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of said channel box toward the control rod side under a condition that a distance L1 between the inner side surface of said channel box positioned on the control rod side and one of said fuel rods positioned at the outermost periphery of said square lattice array and a distance L2 between the inner side surface of said channel box positioned on the anti-control rod side and one of said fuel rods positioned at the outer periphery of said square lattice array satisfy a relationship of L2xe2x88x92L1xe2x89xa70.5 mm.
(15) The present invention provides a reactor core including a plurality of fuel assemblies each of which includes a plurality of fuel rods placed in a square lattice array of 10-rows/10-columns, and at least one control rod inserted among said fuel assemblies, said plurality of fuel assemblies being configured such that a gap between the adjacent two of said plurality of fuel assemblies on the control rod side is larger than a gap between the adjacent two of said plurality of fuel assemblies on the anti-control rod side, wherein at least one of said plurality of fuel assemblies is configured such that the pitch of said plurality of fuel rods is in a range of 12.65 mm to 13.15 mm; the inner width of a channel box is in a range of 133.5 mm to 134.5 mm; and means for offsetting and holding a fuel bundle is provided in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of said channel box toward said channel fastener side.
(16) The present invention also provides a reactor core including a plurality of fuel assemblies each of which includes a plurality of fuel rods placed in a square lattice array of 10-rows/10-columns, and at least one control rod inserted among said fuel assemblies, said plurality of fuel assemblies being configured such that a gap between the adjacent two of said plurality of fuel assemblies on the control rod side is larger than a gap between the adjacent two of said plurality of fuel assemblies on the anti-control rod side, wherein at least one of said plurality of fuel assemblies is configured such that the pitch of said plurality of fuel rods is in a range of 12.65 mm to 13.15 mm; the inner width of a channel box is in a range of 133.5 mm to 134.5 mm, and means for offsetting and holding a fuel bundle is provided in such a manner that the center in a cross section of said fuel bundle is offset from the center in a cross section of said channel box toward the control rod side under a condition that a distance L1 between the inner side surface of said channel box positioned on the control rod side and one of said fuel rods positioned at the outermost periphery of said square lattice array and a distance L2 between the inner side surface of said channel box positioned on the anti-control rod side and one of said fuel rods positioned at the outer periphery of said square lattice array satisfy a relationship of L2xe2x88x92L1xe2x89xa70.5 mm.
(17) The present invention provides a fuel spacer for holding a fuel bundle composed of a plurality of fuel rods placed in a square lattice array of n-rows/n-columns (n: integer) and at least one water rod, said fuel spacer including a band formed into a square shape, and a plurality of tabs projecting outwardly from the outer periphery of said band, wherein the height of those of said plurality of tabs positioned on one side with respect to a diagonal line of the square shape of said band is different from the height of those of said plurality of tabs positioned on the other side with respect to said diagonal line.
(18) The present invention also provides a channel box, formed into an approximately cylindrical shape having a square shape in transverse cross-section, for covering a fuel bundle composed of a plurality of fuel rods placed in a square lattice array of n-rows/n-columns (n: integer) and at least one water rod, said channel box including a plurality of tabs projecting inwardly from the inner periphery of the approximately cylindrical shape of said channel box, wherein the height of those of said plurality of tabs positioned on one side with respect to a diagonal line of the square cross-section of said channel box is different from the height of those of said plurality of tabs positioned on the other side with respect to said diagonal line.